You can get some relatively compact, switch mode chinese made power supplies that are capable of 20 amps from eBay for about $40 (available in 12V, 15V, 24V and etc). They even have a potentiometer to adjust the voltage slightly by about +/- 10%. That's what I did to my car. I got a 15V 20 amp supply, adjusted the output to 14V, then connected the output with a high current diode (of which you can find in an old ATX power supply) in parallel with the car's electrical system and installed everything neatly near the spare tire compartment in the trunk.

Bam, now I can run an extension cord to the car whenever I want to work on the carpc or have the music running while Im in the garage.

That's similar to the one i have but not exactly. It would work though. Just keep the output voltage at 13.5 - 14v and you can simply hook it up in parallel to the car's electrical system. No need to worry about overcharging.

I decided to over-complicate this a little, I wanted to lock out the starter when the car was plugged in, and to require the key to use the computer when the car is plugged in, and automatically disconnect the battery when I plugged the car in. This way I can use this power supply, which is much cheaper, but outputs 12volts. Oh, and I added push-button ignition
Attached is a rough schematic of the system.

Here is my advice... Find yourself some sort of quick connect cable ends that will handle 15VDC @ 30A, wire the female end up directly the car battery (with a suitable fuse) - and mount it so you can access it easily when you need to plug in the car. Under the hood, perhaps. Wire up the male side to a regular car battery charger (~$100 for a nice one, you can probably find one for $25 on eBay or craigslist).

Then, when you want to have the computer on, you just plug the charger in. No need to wire anything else differently. And if you're using a battery isolator, wire up the charge wire to the central post (same one as the alternator) so that both batteries are charged when you plug in.

The reason I decided to do it the way listed above (post 23) is because; a) for the reasons listed there, & b) because I couldn't find a battery charger that could output that high a sustained amperage that was in budget. Take a look at the .txt file attached to that post, I know its crude but you should be able to figure out what is going on.

I tried looking at the .txt file, but it doesn't seem to have formatting - its just a single line of text for me, and I couldn't get it to line up. A .gif or .jpg might be a bit more helpful.

I did a quick search and found a 30 amp battery charger for $80 at Amazon. Even this, I think, is overkill. You had said you needed 30 amps - but that is a LOT. 30 amps @ 12v is 360 watts. 360 watts is enough for my entire home entertainment center, including 5.1 surround sound receiver and speakers, desktop computer, and a 40" LCD (total: 250 watts at idle, 300watts when I'm watching a movie) I would guess that most carpc setups use 150 watts at peak (playing a movie with the speakers blasting) - average around 50W when in use, and idle at 25W. Of course I'm making a lot of assumptions here, it's certainly possible for an audio system alone to consume 500W, they're just rare.

Remember that your charger doesn't need to be providing 100% of the power drain at peak, it only has to provide more than 100% of the average power - in fact, even that isn't necessarily true, as your battery will have some charge in it and you can leave the charger plugged in over night to top it off after you're done.

At the time I posted this thread I was planning on running an OPUS 360, and a midd-low end graphics card. The idea is to be able to run the computer and lights with out any drain on the battery for things like updates, but also at a car show (hence the need to disable the starter). I figured that the computer running at full power playing HD video on at least 3 monitors and all the lights and everything else that would need to be run at a show 30 amps would be adequate. I have since decided to go with a M4-ATX and lower power USB graphics cards, but that is still gonna need a lot of power. Attached is the schematic, this time with Windows formatting instead of UNIX formatting.

I think you're right, with 3 monitors and lights and stuff, you can hit 20 or 30 amps for sure. Also, good call on disabling the starter. It would suck to have somebody try to steal your ride at a show!